The present invention relates to generally a charge transfer device and more particularly a charge transfer device of the type having at least one bend through which the charge transfer channel may be bent by 45.degree., 90.degree. or 180.degree..
As is well known in the art, the charge transfer devices are such that the packet of charge or electrons is transferred through the surface layer of a semiconductive substrate in a predetermined direction. A typical charge transfer device comprises in general a signal input stage into which enters the input signal, a charge-transfer stage and an output stage from which the output signal is derived. In response to the clock pulses applied to the gate electrodes in the charge transfer stage, the signal electrons are transferred.
When the charge transfer channel or line is arranged straight, the peripheral area which is required for interconnections becomes a few times in (two dimensional) area as large as the area of the charge transfer channel including the input and output stages. As a result, the packaging or integration density is considerably reduced. Furthermore as the channel length is increased; that is, as the charge transfer stages are increased in number, the semiconductor substrate must be increased in size accordingly. As a result, the handling of the chip from such substrates becomes inconvenient. In addition, as the semiconductor substrates become large in size and the chip cost high, their mechanical strength is reduced.
In order to overcome the above-described problems, there has been devised and demonstrated a charge transfer device wherein the charge transfer channel is bent many times by a predetermined angle of, for instance, 180.degree.. However the bend is different in construction from the straight charge transfer channel or line section so that various problems arise. As will be described in detail with particular reference to FIGS. 1 through 4 of the accompanying drawings, the bend comprises in general 31/2 charge transfer stages each of which is by far greater in area than the corresponding stages in the straight charge transfer channel section. It is almost next to impossible to reduce the difference in the gate area in charge transfer stage between the bend and the straight charge transfer channel sections to less than 10%. The difference in gate area results in the variation in charge generation between the bend and the straight charge transfer channel or line section. Especially when the charge transfer devices are used as storage means, a decrease in S/N ratio results because of the nonuniform superposition of dark current upon the output signal.
In addition, the channel length of the bend becomes considerably longer than that of the straight charge transfer channel or line section because the gates in the bend are greater in length than those in the straight charge transfer channel or line section. As a result the time for the complete charge transfer becomes different in the bend and the straight charge transfer channel and the transfer efficiency drops.
Moreover the spacing between the adjacent straight charge transfer line sections is so large that a large surface area of the semiconductor substrate is left unused, thus resulting in a low packaging or integration density.